1. Field of the Invention
The present invention relates generally to magnetic disk drives (disk drives), and more particularly to a method of manufacturing a disk drive by using a servo track writer (STW) for measuring the width of the read element to set the write unsafe (WUS) limit.
2. Description of the Related Art
This application is directed to varying an operating parameter known as the write-unsafe limit, or xe2x80x9cWUS limitxe2x80x9d, based on the width of a read element and, in some embodiment, on the width of a write element. As explained below, the WUS limit has historically been fixed for large groups of disk drives without regard to the actual widths of the read and write elements in a given disk drive.
1) An Exemplary Disk Drive and its Read/Write Elements
Referring to FIG. 1, a conventional disk drive 10 has a head disk assembly (HDA) 20 including at least one disk 23, a spindle motor 22 for rapidly rotating the disk 23, and a head stack assembly (HSA) 40 that includes an actuator assembly 50 and a head gimbal assembly (HGA) (not numbered) with a transducer head 80 for reading and writing data. The HSA 40 is part of a servo control system that positions the transducer head 80 over a particular track on the disk to read or write information from that track. The HSA 40 earns its name from the fact that it generally includes a plurality of HGAs that collectively provide a vertical arrangement of heads called a xe2x80x9chead stack.xe2x80x9d
The transducer heads 80 of several years ago were xe2x80x9cmergedxe2x80x9d devices where reading and writing were accomplished with a single inductive element. The transducer head 80 commonly used today, however, is a composite (MR and inductive) transducer head 80 that has separate read and write elements. FIG. 2 is a highly simplified representation of a composite transducer head 80 having it""s a write element 81 of width W and it""s a read element 82 of width R. The transducer head 80 shown is a xe2x80x9cwrite wide, read narrowxe2x80x9d device in that the read element""s width R is typically about 50-65% of the write element""s width W.
Composite transducer heads 80 are very small devices that are manufactured in large batches using photolithographic wafer process techniques. As a result, operating characteristics such as the widths of the read and write elements 81, 82 tend to vary over a normal distribution curve for a given number of heads, wafers or an manufacturers. As explained further below, the wide variability of read width R and write width W is problematic when combined with a fixed WUS limit.
FIG. 3 is an exploded perspective view of a fully-assembled HDA 20 having servo-writing access ports 25, 26 (discussed below) and the controller circuit board 30 that is usually installed after servo-writing. The controller circuit board 30 suitably positions the actuator assembly 50 and then reads or writes user data in accordance with commands from a host system (not shown).
Returning to FIG. 1, the industry presently prefers a xe2x80x9crotaryxe2x80x9d or xe2x80x9cswing-typexe2x80x9d actuator assembly 50 that conventionally comprises an actuator body 51 which rotates on a pivot assembly between limited positions, a coil 52 that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor (VCM), and an actuator arm 54 that extends from the opposite side of the actuator body to support the HGA.
2. An Exemplary Servo Pattern
A disk drive is ultimately used to store user data in one or more xe2x80x9cdata tracksxe2x80x9d that are most commonly arranged as a plurality of concentric data tracks on the surface of its disk or disks. Special servo information is factory-recorded on at least one disk surface so that the disk drive""s servo control system may control the actuator assembly 50, via the VCM, to accurately position the transducer head to read or write user data to or from the data tracks. In colloquial terms, the servo information provides the servo control system with the position of the head relative to the written track. In operation, the disk drive""s servo control system intermittently processes (read only) the pre-recorded servo information just before the disk drive processes (reads or writes) user data in the data tracks.
3. The Write Unsafe Limit
FIGS. 4A, 4B and 4C are data path diagrams that explain why a WUS limit has been used to date and why it is generally set to a small, xe2x80x9cnarrowxe2x80x9d or xe2x80x9ctightxe2x80x9d value when a single WUS limit is used for a family of drives.
FIG. 4A shows a hypothetical data path 501 of a nominally wide write element 81 that is 70% as wide as the track pitch. As shown, the write element 81 settles in along a damped oscillatory path 501 after the servo control system has moved the write element 81 to the desired track in a track seek mode and then entered a track following mode. The WUS limit relates to when writing will be terminated as a function of the oscillatory deviations of the write element""s path 501 relative to track center (T/C). The WUS limit, to put it another way, corresponds to the maximum off-track distance of the write element 81 before writing is disabled. The tighter the WUS limit, the more frequently that writing will be disabled. A higher frequency of disabling writing will reduce the performance of the drive.
The WUS limit is usually specified in terms of a percentage track pitch from track center T/C (e.g. xc2x116%). In FIG. 4A, the write element""s excursions from track center T/C are signified by vertical arrows, varying from +5%, to xe2x88x9210%, to +18%, to xe2x88x9214%, to +5%, to xe2x88x923%. The disk drive""s servo control system stops writing the moment that the write element moves beyond the WUS limit due to resonant vibrations, a shock event, or the like. In FIG. 4A, assuming the WUS limit is set to 16%, and writing is disabled just prior to the 18% excursion. What may not be so apparent from FIG. 4A is that the WUS limit is chosen to minimize or eliminate the detrimental effect of reading erroneous data with a narrow read element. The WUS limit, in more detail, reduces so-called xe2x80x9csliverxe2x80x9d errors, i.e. errors that arise from reading a sliver of old data that remains when new data is written to the same track.
FIG. 4B shows a xe2x80x9cnewxe2x80x9d data path 502. As shown, most of the old data path 501 has been overwritten beneath the new data path 502. Exposed adjacent to the new data path 502, however, are some slivers of old data 501-1, 501-2, 501-3 and 501-4.
FIG. 4C shows a relatively narrow read element 82 attempting to read the data in the new data path 502. This particular read element 82 is represented as being 32% as wide as the data track pitch. As shown, if the data had been written from position xe2x80x9cAxe2x80x9d onward, i.e., with an extremely liberal WUS limit of 33%, the read element 82 may read the old data track slivers 501-1, 501-2 and 501-3 while trying to read the data on the new data path 502. This is completely unacceptable, of course, because it constitutes a data integrity error. There is no resulting ECC error to alert the disk drive""s firmware to the problem. The problem simply goes undetected and the disk drive provides the host with garbled data masquerading as good data.
A WUS limit is useful for preventing sliver errors. The problem, however, is that a single WUS limit is usually applied to an entire family of disk drives even though the width of the read element varies from drive to drive. Under this one size fits all approach, the WUS limit is set to 50% of: (1) the narrowest width of the read elements used in the drive family in order to guarantee that there are no sliver errors; (2) a compromise between (i) an overly-narrow WUS limit that causes too many disk drives to fail during Initial Burn-In (IBI) for repeatedly trying to satisfy the WUS limit and (ii) an overly-wide WUS limit that permits disk drives to pass through IBI with one or more narrow read elements that make the drive susceptible to sliver errors.
The designers choose a narrow WUS limit to eliminate sliver errors from virtually all drives that pass through IBI. Unfortunately, wide read element drives are limited by an unnecessarily narrow WUS limit even though a wider WUS limit could be used for increased performance.
There remains a need for a method of manufacturing a disk drive that allows for variability of the WUS limit in order to enhance the performance of some drives that would otherwise have capability that goes unused.
The invention may be regarded as method of manufacturing a disk drive formed from a head disk assembly (HDA) containing at least one magnetic disk with a magnetic surface and a head stack assembly (HSA) that includes a transducer head with a write element for writing data to the magnetic disk and a read element for reading data from the magnetic disk, the method comprising the steps of: mounting the HDA in a servo track writer and moving the HSA to desired positions over the magnetic disk while writing servo tracks onto the magnetic disk to define a data track pitch; measuring a width of the read element with the servo track writer; and determining a write unsafe (WUS) limit based on the data track pitch and the measured width of the read element, the WUS limit corresponding to a maximum distance during writing that the write element is permitted to move radially offtrack from the centerline of a data track before writing is disabled.
In a more specific context, the step of determining a WUS limit is based on the data track pitch and the measured width of the read element being within a discrete number of predefined width ranges.
In a preferred embodiment of addition, the determined WUS limit is communicated forward for subsequent use by suitable firmware contained in a controller card that is attached to the HDA.